1. Field of the Invention
This invention relates to rubber compositions suitable for coating on steel cords for steel tires.
2. Prior Art
Steel-layered belts, carcasses and chafers, commonly used for radial tires, are generally known as composite structures made up of steel cords and rubbers. In "Polymer Friends", p. 612 (October, 1983), it is proposed to apply by calender coating an unvulcanized rubber sheet of about 0.6 mm thick to both surfaces of a steel cord at a temperature of 80.degree.-120.degree. C. This method however has a drawback in that unvulcanized rubber material adheres to the calender roll and hence becomes difficult to process.
Generally, the extent to which an unvulcanized rubber sheet sticks to the calender roll varies with the difference between the green tack of the rubber to the roll surface and the green strength tending to strip the rubber therefrom. Thin sheets of unvulcanized rubber therefore when exposed at elevated working temperature are literally too sticky to be effectively coated. This is particularly true of rubber compositions in which natural rubber is blended with cobalt naphthenate and phenolic resin so as to attain strong rubber-to-steel adhesion. Such known compositions are susceptible to decline in green strength during calendering because the additives make unvulcanized rubber highly tacky and also because the presence of cobalt ions accelerates the degradation of the rubber.
To cope with this problem, there have been employed tack reducers such as waxes and fatty acids and their esters and metallic salts. Waxes and fatty acid esters are effective for tack reduction at room temperature but not at high temperature. Metallic salts of fatty acids are good at high temperature but cause fast blooming at room temperature which is undesirable for the resulting composite structure to be laminated into a green tire. Fatty acids typified by stearic acid are highly effective for tack reduction at elevated temperature, but to an extent reduce adhesion with steel cords.